Method for estimating nitric acid and nitrous acid ions

ABSTRACT

To estimate nitric acid ions and nitrous acid ions in a sample solution, a method is proposed which comprises the steps of (a) feeding both the sample solution and a reducing agent to an acidic solution which is flowing in a thin pipe; (b) positively mixing the sample solution and the reducing agent and thus reacting the same in a given portion of the pipe; (c) leading the reacted solution to a gas/liquid separator thereby to separate a gaseous phase from the reacted solution; and (d) estimating the nitric acid ions and nitrous acid ions in the gaseous phase.

This application is a continuation of application Ser. No. 08/458,795,filed Jun. 2, 1995 now U.S. Pat. No. 5,668,014.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates in general to devices and methods forestimating the concentration of ionic substances in water, and moreparticularly, to devices and methods for estimating, with an aid of aflow-injection analyzing method, the concentration of ammonium ions (NH₄⁺), nitric acid ions (NO₃ ⁻) and nitrous acid ions (NO₂ ⁻) which are thethree nitrogen-containing ionic substances in water.

2. Description of the Prior Art

For estimating the concentration of the above-mentioned threenitrogen-including ionic substances in water, such as water of rivers,lakes and marshes, various methods have been proposed and put intopractical use, which are for example, ion exchange chromatographymethod, calorimetric method, neutralization titration method and ionelectrode method.

As is known, the ion exchange chromatography method is one of high speedliquid chromatography methods using an ion exchange column, which hasbeen developed particularly for analyzing inorganic anions and cations.That is, the ion exchange chromatography method can precisely estimatethe inorganic anions, such as F⁻, Cl⁻, Br⁻, NO₂ ⁻, NO₃ ⁻, SO₃ ²⁻, SO₄ ²⁻and PO₄₃ ⁻. In this method, a column filled with anion exchange resingrains is used and a sample solution is led into the column from itstop. With this, anions in the sample solution are adsorbed by thegrains. Then, eluate containing a very small amount of competitive anionis led into the column. The amount of the competitive anion is so smallthat a conductivity meter can not detect it. Each anion adsorbed by thegrains competes with the competitive anion and is eluted from the columnwith a certain mobility, so that the concentration of anion in theeluate can be estimated.

According to this ion exchange chromatography method, the ammonium ioncan be estimated to a level of several ppm to several tens of ppm byusing the conductivity meter. For the measurement, about ten minutes areusually needed from the time when the sample solution is led into thecolumn. The quantitative concentration range of this method isrelatively high, that is, from about 0.1 mg/l to about 30 mg/l.

Indophenol blue absorptiometric method is a typical one of thecalorimetric methods. In this method, the indophenol blue which isproduced when ammonium ion, with coexistence of hypochlorous acid ion,reacts with phenol is subjected to an absorbance test in which theabsorbance of the light of 630 nm (nanometer) is measured. Thequantitative concentration range of this method is relatively high, thatis, from about 1.6 mg/l to about 33 mg/l.

In the neutralization titration method, ammonia, which has beenextracted by effecting a distillation, is absorbed by a given amount ofsulfuric acid (viz., 25 m mol/l) to prepare a sample solution, and thesample solution is subjected to a titration test using 50 m mol/l sodiumhydroxide solution to estimate the ammonium ion. The quantitativeconcentration range of this method is relatively high, that is, fromabout 0.3 mg/l to about 40 mg/l.

In the ion electrode method, a sample, which has been subjected to apretreatment, is added with sodium hydroxide solution to prepare asample solution whose pH ranges from about 11 to about 13. With thisprocess, ammonium ions are transformed to ammonia. By using an indicatorelectrode (viz., ammonia electrode), the potential of the samplesolution to estimate the ammonium ions. The quantitative concentrationrange of this method is relatively high, that is, from about 0.1 mg/l to100 mg/l.

As the calorimetric method, a so-called"sulfanilamide-naphthylethyldiamine method" and a so-called"phenoldisulfonic acid method" are also used. In thesulfanilamide-naphthylethyldiamine method, under acidity, sulfanilamideis led into water having nitrous acid ion. With this, the water becomescolored violet due to production of azo-coloring matter in the water.The colored water is then subjected to an absorptiometry to measure theabsorbance of the color. With this, the concentration of nitritenitrogen (NO₂ ⁻ --N) is determined. In the phenoldisulfonic acid method,sulfate is treated with phenoldisulfonic acid to producenitrophenoldisulfornic acid solution which is colored yellow. Thecolored solution is then subjected to an absorptiometry to measure ofthe absorbance of the color. With this, the concentration of nitratenitrogen (NO₃ ⁻ --N) is determined.

However, the above-mentioned methods have various drawbacks which are asfollows.

That is, in case of the ion exchange chromatography method, a relativelong measuring time (above ten minutes) is needed. Furthermore, as apretreatment, filtering of the sample solution is needed for removingsuspended solid and organic substances from the solution. Furthermore,water of rivers, lakes and marshes and sewage water can not be testedcontinuously because speedy treatment can not be made againstcontamination of them.

Usage of two types of ion exchange columns may be thought out. That is,one column is filled with cation exchange resin grains and the othercolumn is filled with anion grains, and by switching the fluid passagesto these two columns, cations and anions in a sample solution areestimated at the same time. However, this method brings about increasein number of movable portions of the equipment and thus increases thepossibility of trouble. Furthermore, replacement of filters has to bemade frequently for cleaning the sample solution.

In case of the calorimetric method, many manual labor operations areneeded due to its inherence. Furthermore, a relatively large amount(viz., about 100 ml) of sample solution (viz., test water) is needed andabout 30 to 60 minutes are needed for the measurement. Thus,automization of this method is very difficult. Furthermore, due to theinherence, this method is can not be used for measuring theconcentration in ppb level.

In case of the neutralization titration method and the ion electrodemethod, troublesome operations are needed and relatively long time isneeded for the measurement. Furthermore, the quantitative concentrationrange of these methods are relatively high, and thus, satisfiedmeasurement has not been obtained in measuring efficiency and measuringaccuracy.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a methodfor estimating the concentration of ammonium ion (NH₄ ⁺), nitric acidion (NO₃ ⁻) and nitrous acid ion (NO₂ ⁻) which are the threenitrogen-containing ionic substances in water, the method being free ofthe above-mentioned drawbacks.

It is another object of the present invention to provide an analyzingdevice by which the method can be smoothly carried out.

According to a first aspect of the present invention, there is provideda method for estimating nitric acid ions and nitrous acid ions in asample solution. The method comprises the steps of (a) feeding both thesample solution and a reducing agent to an acidic solution which isflowing in a thin pipe; (b) positively mixing the sample solution andthe reducing agent and thus reacting the same in a given portion of thepipe; (c) leading the reacted solution to a gas/liquid separator therebyto separate a gaseous phase from the reacted solution; and (d)estimating the nitric acid ions and nitrous acid ions in the gaseousphase.

According to a second aspect of the present invention, there is providedan analyzing device for estimating nitric acid ions and nitrous acidions in a sample solution. The analyzing device comprises a pipe; firstmeans for feeding the pipe with a carrier; second means for feeding thepipe with the sample solution, a reducing agent and clean air; a mixingcoil arranged in the pipe, the mixing coil positively mixing the samplesolution and the reducing agent thereby producing a reacted solution; agas/liquid separator arranged in the pipe downstream of the mixing coil,the separator separating a gaseous phase from the reacted solution; anion detector arranged in the pipe downstream of the separator, the iondetector estimating the nitric acid ions and nitrous acid ions in thegaseous phase; and a recorder for recording the estimated resultsdetected by the ion detector.

According to a third aspect of the present invention, there is provideda method for estimating ammonium ions, nitric acid ions and nitrous acidions in a sample solution. The method comprises the steps of (a) feedingthe sample solution to a carrier which is flowing in a thin pipe; (b)selectively feeding a plurality of reagents to the flowing carrier; (c)positively mixing the sample solution and the reagents one after anotherin a given portion of the pipe thereby producing three types of reactedsolutions in the given portion; (d) leading the reacted solutions to agas/liquid separator thereby to separate a gaseous phase from eachreacted solution; (e) heating each gaseous phase to produce nitrogenmonoxide; and (f) estimating the ammonia ions, the nitric acid ions andthe nitrous acid ions in the three types of nitrogen monoxides produced.

According a fourth aspect of the present invention, there is provided ananalyzing device for estimating ammonium ions, nitric acid ions andnitrous acid ions in a sample solution. The analyzing device comprises apipe; means for feeding the pipe with a carrier; first means for feedingthe carrier with the sample solution; second means for selectivelyfeeding a plurality of reagents to the flowing carrier; a mixing coilarranged in the pipe, the mixing coil positively mixing the samplesolution the reagents one after another thereby to produce three typesof reacted solutions; a gas/liquid separator arranged in the pipedownstream of the mixing coil, the separator separating a gaseous phasefrom each-reacted solution; a heater for heating each gaseous phasethereby to produce nitrogen monoxide; and a detector for estimating theammonia ions, the nitric acid ions and the nitrous acid ions in thethree types of nitrogen monoxides produced.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the present invention will becomeapparent from the following description when taken in conjunction withthe accompanying drawings, in which:

FIG. 1 is a schematic view of an analyzing device for estimatingammonium ion, which is a first embodiment of the present invention;

FIG. 2 is a graph showing a relationship between the temperature of aheating furnace installed in the analyzing device of the firstembodiment and the transform-responsibility with which gaseouscomponents are transformed into nitrogen monoxide;

FIG. 3 is a graph showing a relationship between the concentration ofsodium hypochloride (reagent) and the transform-responsibility;

FIG. 4 is a graph showing a relationship between "pH" of sodiumhypochloride and the transform-responsibility;

FIG. 5 is a graph showing the analytical curve of ammonium ion;

FIG. 6 is a schematic view of an analyzing device for estimating thethree nitrogen-including ionic substances in water, which is a secondembodiment of the present invention;

FIG. 7 is a schematic view showing a first modification of the analyzingdevice of the second embodiment;

FIG. 8 is a view similar to FIG. 7, but showing a second modification ofthe analyzing device of the second embodiment;

FIG. 9 is a schematic view of an automized analyzing system forautomatically estimating the three-nitrogen including ionic substancesin water, which is a third embodiment of the present invention;

FIG. 10 is a schematic view of an analyzing device for estimating nitricacid ions and nitrous acid ions in test water, which is a fourthembodiment of the present invention;

FIG. 11 is a schematic view of a membrane type separator installed inthe analyzing device of the fourth embodiment;

FIG. 12 is a schematic view taken from the direction of the arrow "XII"of FIG. 11;

FIG. 13 is a graph showing a typical signal pattern of chemiluminescencerecorded by a recorder when a test water is tested by the analyzingdevice of the fourth embodiment;

FIG. 14 is a graph showing a relationship between the concentration (N)of sulfuric acid and the intensity (RCL) of chemiluminescence;

FIG. 15 is a graph showing a relationship between the concentration (%)of potassium iodide and the intensity (RCL) of chemiluminescence;

FIG. 16 is a graph showing a relationship between the concentration (%)of titanium trichloride and the intensity (RCL) of chemiluminescence;

FIG. 17 is a graph showing a relationship between the flow rate of cleanair for bubbling and the intensity (RCL) of chemiluminescence;

FIG. 18 is a graph showing the analytical curve of nitrous acid ion andpotassium iodide;

FIG. 19 is a graph showing the analytical curve of nitric acid ion andtitanium trichloride;

FIG. 20 is a first modification of the analyzing device of the fourthembodiment; and

FIG. 21 is a second modification of the analyzing device of the fourthembodiment.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In the following, various embodiments of the present invention will bedescribed in detail with reference to the accompanying drawings.

As is become apparent as the description proceeds, the present inventionis based on a so-called "flow-injection analyzing method" developed in1975 in Denmark by Ruzicka and Hansen and aims to estimate or measurethe quantity of ammonium ion (NH₄ ⁺), nitric acid ion (NO₃ ⁻) andnitrous acid ion (NO₂ ⁻) in water.

In the flow-injection analyzing method, a sample solution and a reagentare continuously injected into a carrier which flows continuously. Thesample solution and the reagent are reacted in a mixing coil. Thereaction product thus produced in the mixing coil is estimated byvarious detectors.

Referring to FIG. 1, there is schematically shown an analyzing device bywhich a method for estimating ammonium ions in test water is carriedout, the method being a first embodiment of the present invention.

In the drawing, denoted by numeral 1 is an inlet opening for both acarrier and a reagent, 2 is an inlet opening for a sample solution and 3is an inlet opening for clean air. In this first embodiment, as areagent led into the carrier inlet opening 1, hypochlorous acid (HOCl)or sodium hypochlorite (NaClO) is used.

Designated by references P₁, P₂ and P₃ are pumps, 4 is a mixing coil and5 is a gas/liquid separator. The separator 5 is provided with a cleanair inlet opening 5a and a waste liquid outlet opening 5b. The separator5 comprises a glass tube in which a gas permeable membrane is arranged.As shown, the separator 5 is inclined at a given degree "θ" relative tothe horizontal surface. The separator 5 is rotated about its axis by apower unit (not shown).

Designated by numeral 6 is a source of clean air, 7 is a heatingfurnace, 8 is a detector, 14 is an ozonizer and 9 is a recorder. In thefirst embodiment, as the detector 8, a chemiluminescent detector isused. The parts are connected by a thin pipe 10 in the illustratedmanner. The thin pipe 10 is constructed of Teflon (trade name) whoseinner diameter is about 0.5 mm to 1.0 mm.

By the mixing coil 4, there is produced a turbulent flow by which amixing and thus a reaction between the sample solution and the reagentis smoothly carried out. In fact, the mixing coil 4 is a coiled thintube of Teflon (trade name). The diameter of the tube is about 0.5 mm to1.0 mm. The length of the coil is determined in accordance with themixing ability which is required by a user.

For obtaining a desired repeatability, a rotary valve type injector (notshown) is used to feed the sample solution into the inlet opening 2. Asthe pumps P₁, P₂ and P₃, BERRY-STAR TYPE pumps are used, each having apumping capacity of about 0.1 to several ml/min.

In the following, a process for estimating ammonium ions in a test waterby using the above-mentioned flow-injection analyzing device of FIG. 1will be described, which is the first embodiment of the presentinvention.

Under operation of the pump P₂, the test water containing the ammoniumions is led into the sample solution inlet opening 2, and underoperation of the pump P₁, hypochlorous acid (HOCl) (or sodiumhypochlorite (NaClO)) is led into the reagent inlet opening 1. Withthis, they are mixed at point "A" of the pipe 10. For increasing themixed condition of them at point "B", clean air is led into the cleanair inlet opening 3 under operation of the pump P₃.

The ammonium ions and the hypochlorous acid are thus mixed sufficientlyin the mixing coil 4 and reacted and then led into the gas/liquidseparator 5. Due to feeding of clean air into the separator 5 from theclean air source 6, the gas separation from the liquid phase in themixing coil 4 is promoted. The gaseous components from the separator 5are led into the heating furnace 7, while a waste liquid from the wasteliquid outlet opening 5b of the separator 5 is led to a waste liquidtank 11.

When heated by the heating furnace 7, the gaseous components aretransformed into nitrogen monoxide (NO), and the sample gas thuscontaining the nitrogen monoxide (NO) is led into the detector 8 intowhich ozone gas from the ozonizer 14 is being fed. Thus, the nitrogenmonoxide (NO) in the sample gas is reacted with the ozone gas (O₃) toproduce a chemiluminescence whose intensity is detected by the detector8. With this, the concentration of the nitrogen monoxide (NO) in thesample gas is measured and recorded by the recorder 9.

By using the above-mentioned process, many tests were carried out by theinventors and the results of them are depicted in FIGS. 2 to 5.

FIG. 2 is a graph showing the relationship between the temperature ofthe heating furnace 7 and the transform-responsibility with which thegaseous components are transformed into nitrogen monoxide (NO). As isunderstood from this graph, the transform-responsibility showed asufficient value at about 600° C.

FIG. 3 is a graph showing a relationship between the concentration(mol/l) of sodium hypochlorite (NaClO,pH7) and thetransport-responsibility. As is known, the sodium hypochlorite (NaClO)is a reagent for reducing ammonium ions in water to nitrogen monoxide(NO). As is understood from this graph, the transform-responsibilityshowed the maximum value at about 0.02 (mol/l) concentration of thesodium hypochlorite. In case of hypochlorous acid (HOCl) as the reagent,substantially the same result was obtained.

FIG. 4 is a graph showing a relationship between "pH" of sodiumhypochloride (NaClO) and the transform-responsibility, from which the"pH" dependency of the sodium hypochloride with respect to thetransform-responsibility is known. As is understood from this graph, thetransform-responsibility showed the maximum value at about pH5 to pH8(viz., near neutral).

Considering this result, for providing the analytical curve of ammoniumions, tests were carried out under a condition wherein the concentrationof the sodium hypochlorite (NaClO) was 0.02(mol/1) and the "pH" of thesame was about 5 to 8. The curve is shown by the graph of FIG. 5. As isseen from this graph, the analytical curve showed a desired linearity(or desired transform-responsibility) at a relatively low concentrationrange, that is, from 1 ppb (=μg/l) to 1500 ppb (=1.5 mg/l) of theammonium ions. Furthermore, the analytical curve had a satisfaction inthe standard deviation and the variation coefficient.

Referring to FIG. 6, there is schematically shown an analyzing device bywhich a method for estimating the three nitrogen-including ionicsubstances in water is carried out, the method being a second embodimentof the present invention. As has been mentioned hereinabove, the threenitrogen-including ionic substances in water are ammonium ion (NH₄ ⁺),nitric acid ion (NO₃ ⁻) and nitrous acid ion (NO₂ ⁻).

In FIG. 6, for ease of understanding, parts or portions similar to thoseof the above-mentioned first embodiment (FIG. 1) are denoted by the samereference numerals.

In the drawing, denoted by references, 1b and 1c are inlet openings forboth the carrier and the reagents, 2 is an inlet opening for a samplesolution and 3 is an inlet opening for clean air. In this secondembodiment, as a reagent led into the first reagent inlet opening 1a,titanium trichloride (TiCl₃) is used, as a reagent led into the secondreagent inlet opening 1b, potassium iodide (KI) is used, and as areagent led into the third reagent inlet opening 1c, hypochlorous acid(HOCl) is used.

Designated by references P₂ to P₇ are pumps, and 12 is a mixing valvefor each reagent. Designated by numeral 10 is a pipe for connecting thevarious parts in the illustrated manner. 4 is a mixing coil, 5 is agas/liquid separator, 7 is a heating furnace, 13 is a dryer, 8 is adetector, 9 is a recorder and 14 is an ozonizer. As the detector 8, areduced pressure type chemiluminescent detector is used.

In the following, a process for estimating the three nitrogen-includingionic substances in a test water by using the analyzing device of FIG. 6will be described, which is the second embodiment of the presentinvention.

First, a process for estimation of nitric acid ion (NO₃ ⁻) will bedescribed.

Under operation of the pump P₂, the test water containing nitric acidions is led into the sample solution inlet opening 2, and underoperation of the pump P₄, titanium trichloride (TiCl₃) as a reagent isled into the first reagent inlet opening 1a. Then, the mixing valve 12is manipulated to mix them. Then, under operation of the pump P₃, cleanair is fed into the clean air inlet opening 3, the mixed components meetthe clean air at point "C" and they are led to the mixing coil 4. Thetest water and the reagent are thus mixed sufficiently in the mixingcoil 4 to achieve a sufficient reaction therebetween. Then, they are ledinto the gas/liquid separator 5. The gaseous components from theseparator 5 are led into the heating furnace 7, while a waste liquidfrom the separator 5 is discharged through the outlet opening 5b.

When heated at about 600° C., the gaseous components are transformedinto nitrogen monoxide (NO), and the sample gas thus produced is ledinto the dryer 13 and then led to the detector 8 under operation of thepump P₇. The nitrogen monoxide (NO) in the gaseous phase is reacted withthe ozone gas (O₃) to produce a chemiluminescence whose intensity isdetected by the detector 8 and recorded by the recorder 9.

The chemi-luminescence detector 8 is of a NOx type which can measurenitrogen oxides. The chemi-luminesecence is the luminous phenomenonwhich occurs when excited molecules are returned to the ground state. Byanalyzing the emission spectrum, a qualitative analysis of the sample isachieved, and by measuring the luminous energy, a quantitative analysisis achieved. That is, the chemi-luminescence detector 8 used in thisembodiment is of a type which practically uses the fact wherein theintensity of the chemi-luminescence produced from the reaction betweenNO in the sample gas and O₃ is substantially proportional to theconcentration of NO. Thus, the concentration of the nitrogen monoxide(NO) in the sample gas is measured for estimating the nitric acid ion(NO₃ ⁻) in water.

Now, a process for estimation of nitrous acid ion (NO₂ ⁻) will bedescribed.

Similar to the above case, under operation of the pump P₂, the testwater containing nitrous acid ions is led into the sample solution inletopening 2, and under operation of the pump P₅, potassium iodide (KI) asa reagent is led into the second reagent inlet opening 1b. Then, themixing valve 12 is manipulated to mix them. Then, under operation of thepump P₃, clean air is fed into the clean air inlet opening 3, the mixedcomponents meet the clean air at point "C" and they are led to themixing coil 4 and reacted there. Then, they are led into the gas/liquidseparator 5. The gaseous components from the separator 5 are led intothe heating furnace 7, while a waste liquid from the separator 5 isdischarged through the outlet opening 5b.

Thereafter, like in the case of the above-mentioned nitric acid ions, achemiluminescence produced by the reaction between the nitrogen monoxide(NO) in the gaseous phase and the ozone gas (O₃) from the ozonizer 14 ismeasured by the detector 8 and recorded by the recorder 9.

As is understood from the above, in the second embodiment, a reagent isfed to a test water to reduce nitric acid ions (NO₃ ⁻) or nitrous acidions (NO₂ ⁻) to nitrogen monoxide (NO) and the nitrogen monoxide isreacted with ozone gas (O₃). That is, since the detector 8 is of thechemi-luminescent detector which is designed to measure the intensity ofthe luminescence produced when NO and O₃ are reacted, such reduction isnecessary for estimating the nitric acid ions (NO₃ ⁻) or nitrous acidions (NO₂ ⁻) to nitrogen monoxide (NO).

The nitric acid ion (NO₃ ⁻) has three oxygen atoms while the nitrousacid ion (NO₂ ⁻) has two oxygen atoms. Thus, the nitrous acid ion (NO₂⁻) is easily reduced to the nitrogen monoxide (NO) as compared with thenitric acid ion (NO₃ ⁻). By using this difference, in the secondembodiment, two reducing agents are employed, one being the potassiumiodide (KI) which can reduce the nitrous acid ion (NO₂ ⁻) only, and theother being the titanium trichloride (TiCl₃) which can reduce both thenitrous acid ions (NO₂ ⁻) and the nitric acid ions (NO₃ ⁻). The reactionformulas of them are shown in the following.

    NO.sub.3.sup.- +3Ti.sup.3+ →NO+3Ti.sup.4+           (1)

    2NO.sub.2.sup.- +2I.sup.- →2NO+I.sub.2              (2)

While, in case of estimating ammonium ion (NH₄ ⁺), the test watercontaining ammonium ions is led into the sample solution inlet opening2, and under operation of the pump P₆, hypochlorous acid (HOCl) is ledinto the third reagent inlet opening 1c. Thereafter, similar operationsare carried out. With this, the ammonium ions are transformed into thenitrogen monoxide (NO) which is thereafter subjected to thechemi-luminescence analysis as mentioned hereinabove.

Although, in the above description in the second embodiment, the processfor estimating the three nitrogen-including ionic substances (viz., NH₄⁺, NO₃ ⁻ and NO₂ ⁻) is explained separately, these three substances canbe continuously estimated or measured. For this continuous measurement,the three reagents (viz., titanium trichloride, potassium iodide andhypochlorous acid) are intermittently fed to a sample solution whichcontinuously flows in the tube 10. In this case, the detector 8 issuescontinuously three types of outputs, one being an output proportional tothe concentration of nitrous acid, one being an output proportional tothe concentration of nitrous acid and nitric acid and the other being anoutput proportional to the concentration of ammonia.

FIG. 7 shows a first modification of the above-mentioned secondembodiment.

In this modification, a filter 17 is further employed, which cleans thetest water 15 before the latter is subjected to the substance estimatingprocess. A filter medium 16 is packed in the filter 17. For cleaning thefilter medium 16, a clean water pipe 20 is connected to the bottom ofthe filter 17. Filtered water from the filter 17 is led to the analyzingdevice 19 through a pipe 18. In fact, this modification is very usefulto examine water which is contaminated with suspended solids.

FIG. 8 shows a second modification of the second embodiment. In thismodification, a ultrafiltration film or hollow fiber is used as thefilter medium of the filter 17'.

Referring to FIG. 9, there is shown an automized analyzing system whichcan automatically estimate the three-nitrogen including ionic substancesin water, the system being a third embodiment of the present invention.For ease of understanding, parts or portions similar to those of theabove-mentioned first and second embodiments (FIGS. 1 and 6) are denotedby the same reference numerals.

In the system of this third embodiment, near the inlet opening "A" ofthe pipe 10 for the reagent, there are arranged three tanks (not shown)for hypochlorous acid (HOCl) (or sodium hypochlorite (NaClO)), titaniumtrichloride (TiCl₃) and potassium iodide (KI). From the tanks, thereextend respective feeding pipes 1a, 1b and 1c which are connected to theinlet opening "A" of the pipe 10 through a reagent feeding mechanism 23which comprises a pump P₈ and two electromagnetic valves V₁ and V₂. Thevalves V₁ and V₂ are used for selectively feeding the three reagents tothe pipe 10. Designated by numeral 22 is a control unit which isinterposed between the reagent feeding mechanism 23 and the detector 8.

In operation of the automized analyzing system, under operation of thepump P₉, test water is fed to the pipe 10 from the sample solution inletopening 2 and at the same time clear air is fed to the pipe 10 from theclean air inlet opening 3 while adjusting an adjusting valve V₃.Thereafter, under operation of the pump P₈, ON-OFF operation of the twovalves V₁ and V₂ is selectively carried out so as to feed the threereagents (viz., titanium trichloride (TiCl₃), potassium iodide (KI) andhypochlorous acid (HOCl)) to the pipe 10 independently one afteranother.

Like the above-mentioned cases, the test water and the reagent solutionare mixed at the mixing coil 4, and then they are led to the gas/liquidseparator 5. The gas components from the separator 5 are led to theheating furnace 7 and transformed into nitrogen monoxide (NO). Thenitrogen monoxide (NO) is thereafter subjected to the chemi-luminescenceanalysis in such a manner as has been mentioned hereinafore. Byanalyzing information signals from the reagent feeding mechanism 23 andthe detector 8, the control unit 22 calculates the sum of the threenitrogen-including ionic substances based on the analytic curvesprovided by them.

Referring to FIG. 10, there is schematically shown an analyzing deviceby which a method for estimating nitric acid ion (NO₃ ⁻) and nitrousacid ion (NO₂ ⁻) in test water is carried out, the method being a fourthembodiment of the present invention.

In the drawing, denoted by reference P is a pump, 1 is an inlet openingfor a carrier, 2 is an inlet opening for a sample solution and 3 is aninlet opening for clean air. In this fourth embodiment, as the carrierled into the carrier inlet opening 1, 1N-sulfuric acid is used. Forsmoothly feeding the sample solution into the sample solution inletopening 2, a rotary valve type injector is used. The injection rate isfrom about 10 μl to about 100 μl. As the pump P, a BERRY-STAR TYPE pumpis used whose pumping capacity is about 0.1 to several ml/min.

Designated by reference 1a is an inlet opening for a reagent, 4 is amixing coil and 5 is a membrane type separator. The separator 5 isprovided with a clean air inlet opening 5a through which clean air froma clean air source 6 is led into the separator 5. The mixing coil 4 is acoiled thin tube of Teflon (trade name) whose diameter is about 1.0 mmand whose length is about 200 cm.

Denoted by numeral 8 is a detector, that is, a chemi-luminescencedetector and 9 is a recorder. These parts are connected by a pipe 10which is constructed of Teflon (trade name). Preferably, the innerdiameter of the pipe 10 is about 0.5 mm to 1.0 mm.

As is seen from FIGS. 11 and 12, the membrane type separator 5 comprisesinner and outer cylinders 5b and 5c which are coaxially arranged. Theinner cylinder 5b is a perforated tube of Teflon (trade name) whoseinner diameter is 2 mm and whose length is 20 cm. While, the outercylinder 5c is a nonpermeable tube of Teflon (trade name) whose innerdiameter is 4 mm and whose length is 20 cm. The clean air inlet opening5a is connected to the interior of the inner cylinder 5b. The outercylinder 5c is equipped with both an inlet opening 5d into which themixed and reacted solution from the mixing coil 4 is led and a wastedischarge opening 5e from which a waste liquid separated from the mixedand reacted solution is discharged.

The chemi-luminescence detector 8 is of a NOx type which can measurenitrogen oxides. More specifically, the detector 8 employed in thefourth embodiment is the "238-type" detector produced in KIMOTO DENSHICO., LTD.

In the following, a process for estimating the nitric acid ions (ornitrous acid ions) in a test water by using the analyzing device of FIG.10 will be described.

Under operation of the pump P, an acidic solution including 1N-sulfuricacid (carrier) is led into the carrier inlet opening 1. With the acidicsolution being led into the opening 1, a test water is led into thesample solution inlet opening 2, and at the same time, a reducer of 100μl of titanium trichloride (or 100 μl of potassium iodide in case ofnitrous acid ions) is led into the reagent inlet opening 1a. With this,the test water and the reducer meet at the point "A" and thensufficiently mixed at the mixing coil 4 and reacted. For promoting themixing and thus reaction of them, clean air is led into the point "B"from the clean air inlet opening 3.

Then, the reacted solution from the mixing coil 4 is led into themembrane type separator 5 through the inlet opening 5d. In the separator5, nitrogen monoxide (NO) is separated from the reacted solution and ledinto the detector 8 together with the clean air from the clean airsource 6. The waste liquid from the separator 5 is discharged to a wasteliquid tank (not shown) through the waste discharge opening 5e.

The intensity of the chemiluminescence, which has been produced by thereaction between nitrogen monoxide (NO) in the gaseous phase and ozonegas (O₃), is detected by the detector 8 and recorded by the recorder 9.

FIG. 13 shows a typical signal pattern recorded by the recorder 9. Bymeasuring the height "h" of the pattern, the concentration of "NOx" isknown.

In this fourth embodiment, the following reduction reactions are carriedout like in the above-mentioned second embodiment.

That is:

    NO.sub.3.sup.- +3Ti.sup.3+ →NO+3Ti.sup.4+           (1)

    2NO.sub.2.sup.- +2I.sup.- →2NO+I.sub.2              (2)

These reduction reactions show a higher reaction efficiency under acidiccondition, and the titanium trichloride tends to produce precipitatesunder a condition other than the acidic condition. The precipitates maylower the performance of the membrane type separator 5. Accordingly,1N-sulfuric acid is used as the carrier. If, in place of sulfuric acid,hydrochloric acid or nitric acid is used as the carrier, sufficientintensity detection for chemiluminescence can not be expected by thedetector 8 because they may permeate the membrane type separator 5.Phosphoric acid can not be used as the carrier because it producesinsoluble materials with titanium trichloride.

By using the above-mentioned process, many tests were carried out by theinventors and the results of them are depicted FIGS. 14 to 19.

FIG. 14 is a graph showing a relationship between the concentration (N)of sulfuric acid (carrier) and the intensity (RCL) of the producedchemiluminescence. The concentration of nitrous acid ions or nitric acidions in the test water was 1.4 ppm and the concentration of potassiumiodide was 10% and the concentration of titanium trichloride was 20%.1N-sulfuric acid was used as the carrier. It was found that if theconcentration of sulfuric acid was lower than 1N, the reductionefficiency was lowered.

FIG. 15 is a graph showing a relationship between the concentration (%)of potassium iodide and the intensity (RCL) of the producedchemiluminescence. As is seen from this graph, when the concentration ofpotassium iodide was from 0.1% to 20%, a constant intensity ofchemiluminescence was obtained. About 10% is preferable.

FIG. 16 is a graph showing a relationship between the concentration (%)of titanium trichloride and the intensity (RCL) of the producedchemiluminescence. As is seen from the graph, the intensity increasedwith increase of the concentration of titanium trichloride. As isunderstood from the graph, when the concentration of titaniumtrichloride solution is greater than 5%, satisfactory intensity ofchemiluminescence is produced. Preferably, the concentration is about20%.

FIG. 17 is a graph showing a relationship between the flow rate of cleanair fed to the clean air inlet opening 3 (see FIG. 10) and the intensity(RCL) of the produced chemiluminescence. As is seen from the graph,about 6 ml/min is preferable in the flow rate.

FIG. 18 is a graph showing the analytical curve of nitrous acid ions(NO₂ ⁻) and potassium iodide (KI), and FIG. 19 is a graph showing theanalytical curve of nitric acid ions (NO₃ ⁻) and titanium trichloride(TiCl₃). As is seen from these graphs, in case of FIG. 18, theanalytical curve showed a desired linearity (viz., desiredtransform-responsibility) at the range from 7 ppb to 1.4 ppm, while, incase of FIG. 19, the analytical curve showed a desired linearity (viz.,desired transform-responsibility) at the range from 7 ppb to 14 ppm.

The accuracy of the measurement was as follows. That is, whenmeasurement was carried out five times repeatedly on nitrous acid ionsof 0.28 ppm, the coefficient of variation was 1.4%, and when similarmeasurement was carried out on nitric acid ions of 0.14 ppm, thecoefficient of variation was 1.2%.

In order to check the accuracy of the method of the above-mentionedfourth embodiment, a conventional calorimetric method and a conventionalion exchange chromatography method were also carried out together withthe method of the fourth embodiment on four types of test water. Theresults are shown in TABLE-1 and TABLE-2.

                  TABLE 1                                                         ______________________________________                                                  NO.sub.2.sup.-  (ppb)                                               Test water  Present Invention                                                                         Colorimetic Method                                    ______________________________________                                        Ujigawa     64          66                                                      Katsuragawa 37 39                                                             Kizugawa 15 14                                                                Kamogawa 32 34                                                              ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                                  NO3- (ppm)                                                          Test water  Present Invention                                                                         Ion Exc. Chromato.                                    ______________________________________                                        Ujigawa     1.4         1.4                                                     Katsuragawa 4.2 4.3                                                           Kizugawa 2.2 2.0                                                              Kamogawa 2.0 2.0                                                            ______________________________________                                    

As is seen from these tables, the method of the invention and theconventional methods showed substantially the same results.

Referring to FIG. 20, there is shown a first modification of theanalyzing device of the above-mentioned fourth embodiment.

In this modification, a countercurrent type "NO" gas separator 5 isemployed in place of the membrane type separator 5. Similar to themembrane type separator 5, the countercurrent type "NO" gas separator 5comprises a reacted solution inlet opening 5d, a clean air inlet opening5a, a waste discharge opening 5e and a NO gas outlet opening 5f. In theseparator 5, during the time when the reacted solution from the mixingcoil 4 is flowing down along an outer surface of a tube, the clean airis led into the tube from the clean air inlet opening 5a. With this, NOgas is discharged from the reacted solution and led to the detector 8.

Referring to FIG. 21, there is a second modification of the analyzingdevice of the above-mentioned fourth embodiment.

In this modification, connected to the reagent inlet opening 1a of thepipe 10 is a reagent feeding mechanism 23. The mechanism 23 comprises atank 24 for potassium iodide (KI) and a tank 25 for titanium trichloride(TiCl₃) which are connected to the reagent inlet opening 1a through athree-way electromagnetic valve 26. Designated by numeral 22 is acontrol unit.

In operation, by manipulating the valve 26, the potassium iodide (KI)and the titanium trichloride (TiCl₃) are selectively fed to the reagentinlet opening 1a. With this, based on the above-mentioned measuringprocess, the nitric acid ions (NO₃ ⁻) and the nitrous acid ions (NO₂ ⁻)can be continuously estimated.

What is claimed is:
 1. A method for estimating selectively both nitricacid ions and nitrous acid ions in a sample solution, by using a commonanalyzing device, comprising the steps of:(a) feeding both the samplesolution and a reducing agent to an acidic solution which is flowing ina pipe; (b) feeding clean air to the acidic solution which has thesample solution and the reducing agent contained therein; (c) positivelymixing the sample solution and the reducing agent under existence of theclean air in a mixing coil installed in said pipe thereby to produce areacted solution; (d) leading the reacted solution to a membrane typegas/liquid separator to separate a gaseous phase from the reactedsolution, said separator being inclined by an angle "θ" with respect toa horizontal line and arranged to be rotatable about an axis of saidseparator; and (e) estimating the nitric acid ions and nitrous acid ionsin the gaseous phase, the estimation being made by usingchemiluminescence detection technique, wherein at the step (a), greaterthan 5% titanium trichloride solution is used as the reducing agent whenthe ions which are to be estimated are nitric acid ions, and wherein0.1% to 20% potassium iodide solution is used as the reducing agent whenthe ions which are to be estimated are nitrous acid ions.
 2. A method asclaimed in claim 1, wherein said separator is of a countercurrent type.3. A method as claimed in claim 1, wherein the concentration of saidtitanium trichloride solution is 20%.
 4. A method as claimed in claim 1,wherein said acidic solution flowing in said pipe is a sulfuric acidsolution.
 5. A method as claimed in claim 1, wherein said acidicsolution flowing in said pipe is 1N to 2N-sulfuric acid solution, theconcentration of said titanium trichloride solution is 20%, and theconcentration of said potassium iodide solution is 10%.
 6. A method asclaimed in claim 1, wherein at least one of the steps (a),(b),(c),(d)and (e) are automatically carried out.